200 Siluria. | April, 
“old school” recording his best arguments in favour of its prin- 
ciples; and we feel sure that, whatever can be said—for it has been 
stated by him at least as well as, probably much better than, it 
could be done by anyone else. One consideration we will venture 
to suggest to the readers of this review, and of his book :—If 
modern causes have operated for a certain definite time on the 
crust of the earth, their effect may be conventionally represented 
by a certain symbol; if for twice that period, then by a symbol of 
twice the value; and so on. Now the Silurian deposits are very 
many times more ancient than (say) the Hocene ; and if the intensity 
of modern causes acting since the Eocene period has been sufficient 
to metamorphose Eocene strata into gneiss, to upheave them, and 
even to overturn them, how very much greater must be the effect of 
the same causes, continuously acting for such an immense period 
upon the Silurian deposits, which are so many times older than the 
Eocene. 
These general considerations, however, form merely the author’s 
peroration, and in no way affect the general value of the book, 
which is a perfect storehouse of facts carefully observed, and gar- 
nered for the use of the present and future generations of geologists. 
